An inertial impactor has one or more nozzles which direct a jet of gas, such as air, and particles carried by the gas to an impaction plate. The particles larger than the cut-off size of the impactor cross the streamlines and impinge upon the impaction plate. The smaller particles pass with the gas stream out of the impaction region.
Cascade impactors have been used for a number of years to collect aerosol particles. A cascade impactor has a series of nozzles with successive nozzle orifices being smaller. The smaller the nozzle orifices, the higher the velocity of gas and particles moving through the orifices. The size range of the particles collected on the impaction plate is the function of the velocity of the particles moving through the nozzle orifices. The higher the velocity, the smaller the particles that are collected on the impaction plate. The deposits of particles of these impactors are normally analyzed by microscope inspection, by weighing, or by chemical analysis to determine the chemical composition of the particles.
In general, it is the current practice to disassemble the impactor piece-by-piece each run and remove the impaction plates. New impaction plates are installed. The impactor is then reassembled piece-by-piece. This is an objectionable procedure, as the impaction plate for each stage of the impactor must be handled on disassembly and transferred to some container for further analysis. The assembly and disassembly operations are tedious and time consuming, making the use of the impactor difficult in field operations. If small deposits of particles are collected for gravimetric or chemical analysis, there is a very high probability that some of the deposits will be damaged in the changing of the impaction plates. In the cascade impactor the damage to the deposit of particles on one impaction plate would make it impossible to determine the correct size distribution of the particles.
One of the major difficulties in using impactors is that the particles which impact on an impaction plate may bounce off or be blown off the plate and re-entrained by the gas stream. The particles in a cascade impactor will then not be collected in their proper impaction plate, but will be collected on an impaction plate intended to collect smaller particles. This causes erroneous results to be obtained from an analysis of the deposited particles. Particle bounce increases as the quantity of particles collected under the nozzle increases. A uniform deposit over the entire impaction plate is a desirable feature in reducing particle re-entrainment.
One method used to reduce particle bounce or re-entrainment is to coat the impaction plate with a sticky substance. Once a uniform layer of particles has been collected on the sticky substance, the particles which impact on the impaction plate will bounce from the previously collected particles and not come in contact with the sticky substance. Another method which is used to reduce the bounce of particles is to move the impaction plate each time a particle impaction area becomes loaded with particles or to continuously move the impaction plate. A slotted cascade impactor, the Lundgren impactor, impacts the particles on the surface of a rotating drum. There is no attempt to have uniform deposit of particles on the impaction surface of the drum. Another method to reduce bounce is to move a glass microscopic slide relative to the nozzle. The purpose of moving the slide is to obtain a time resolution of the particles being sampled.
In the field of aerosol science it is advantageous to be able to determine the elemental analysis of the particles which are collected on the impaction plate. One device which is used in this type of study is the x-ray fluorescent analyzer. It is desirable that particles be distributed uniformly in the area illuminated by x-ray, which is typically about 4 cm in diameter. One method of uniformly distributing the particles is filtering the particles from the air stream by a filter. Size classification of particles can be obtained by passing the particles through a dichotomous impactor which divides the particles into two size ranges in the airborne state. The particles are then filtered from the air streams. This classifies the particles into two ranges, one larger and one smaller than the cut-off size of the impactor. Cascade impactors deposit particles in an uneven distribution. Thus, the deposits of particles on the plates of a conventional cascade impactor are not feasible for use with the x-ray fluorescent analyzer. However, if a cascade impactor would collect particles uniformly on the impactor plate, x-ray fluorescent analysis of the deposits would be uniform.